Battery selection--a vital element in the success of AMR projects.
Currently, meter manufacturers and their customers are pushing for AMR systems designed to last longer and longer. In fact, unless local certification rules say otherwise, they are now looking for maintenance-free meters, equipped with batteries that last accordingly. Where battery lifetimes of five to 10 years were once considered as entirely satisfactory, the envelope is being extended to 15 to 20 years, even in the most demanding environmental conditions.
Furthermore, new developments in AMR such as the advent of AMM (automated meter management) deployments with two-way, real-time, communication between utilities and their customers--place an ever increasing demand on the batteries that power the new generation of "smart" meters. So now, rather than having to supply pulses of just a few milliamps to support radio communication over a distance of a few meters, the battery might have to sustain a much higher level of performance to supply a couple of amps to power a GPRS transceiver.
AMR's main role is usually to provide a communication link to enable gas, water, electricity or heat utility meters to be read and billed without the need to gain access to a customer's premises. These meters are "read" either by a hand-held terminal or by the drive-by method in which the data is received by a mobile receiver in a reading vehicle.
Now, though, there is a new generation of AMR that places the meter in direct, constant, two-way communication with the utility. So instead of using AMR just for billing purposes, the utility can take advantage of AMM (automated meter management) as a key tool to help manage its customer relationships by providing complete transparency regarding consumption, tariff and usage profile.
AMM Communication Architectures
In general, there are two competing system architectures for AMM deployments. First, there is the "one-stop" arrangement in which a single meter per customer provides either a low-power, short-range radio signal that is read either by walk-by/drive-by or a high power signal carried by the GSM/CPRS mobile communications network. Then there is the "two-step" arrangement in which up to 100 meters are networked by hard-wired connection to a data concentrator that provides the two-way communication with the utility control center, usually by GSM/GPRS.
When specifying an AMR system, battery selection becomes critical. The first question "which battery chemistry to use?" is simple to answer. The need for high voltage, high capacity, high current pulses and long operating life in demanding operating conditions including extreme temperatures, humidity, dust and UV means that, currently, a primary lithium battery has to be used.
Lithium is the lightest of all metals, and exhibits an exceptional specific capacity (3.86 Ah/g) and unique electrochemical characteristics. Combining lithium with manganese dioxide (Mn[O.sub.2]) powder, polycarbon monofluoride (CFx) or low-freezing point liquid cathode materials, such as thionyl chloride (SO[Cl.sub.2]) or sulfur dioxide (S[O.sub.2]) results in primary cells with high energy, low weight, reduced self-discharge and the ability to operate under extreme conditions such as temperatures ranging from -40[degrees]C to +95[degrees]C.
Li-SO[Cl.sub.2] (lithium thionyl chloride) cells have a nominal voltage of 3.6 V, while both Li-S[O.sub.2] and Li-Mn[O.sub.2] ceils have a nominal voltage of 2.9 V Li-CFx cells have a nominal voltage of 2.8 V. They are not normally used in AMRs for data transmission applications, but may be used to provide memory backup.
Of the available lithium battery chemistries, Li-SO[Cl.sub.2] ceils offer the best choice for AMR applications since they combine high energy density (typically 1,200 Wh/1), excellent temperature characteristics, low self-discharge rates and excellent safety characteristics. This type of cell is offered by approximately 20 manufacturers worldwide.
The higher cell voltage is a further advantage since it could enable one battery to be used rather than two lower voltage cells, and ensures compatibility with electronic circuitry which may need a minimum of say, 2.9 V, to function. This is particularly important as the voltage of lithium cells becomes depressed at lower temperatures, so in an outdoor installation a 3.0 V nominal system could fall below 2.9 V. Li-SO[Cl.sub.2] cells are generally available in a range of sizes from 1/2AA up to DD.
Ambient operating temperatures vary considerably according to location. For instance, gas meters in outdoor applications can routinely experience temperatures as low as -30[degrees]C, while a water meter fixed in a utility room, close to a boiler, can be subjected to constant temperatures of +50[degrees]C. Both these temperatures are well within the Li-SO[Cl.sub.2] cell's normal operating range, but prolonged exposure will impinge on its performance and operating life.
In addition to the nominal battery voltage and cut-off voltage, identifying the pulse profile--cut-off voltage, current, duration and frequency--that has to be sustained is crucial to identifying the right battery for the application. If the AMR utilizes low-power RF, with a required reading range of just a few meters, then the base current drawn during its "sleep" mode will be just a few microamps with pulses during data reading and transmission of around 10 milliamps for a few milliseconds at intervals ranging from hourly to just a few times per day.
The demand on a battery for GSM/GPRS is significantly different as more power is required to transmit a signal to a receiver several hundred meters distant--or many thousand meters above if communicating directly with a satellite. This requires the battery to provide a current pulse ranging from 500 milliamps to 2 A for a few milliseconds.
Battery suppliers often highlight the low "self-discharge" capability of their lithium ceils, referring to the low loss of capacity during shelf storage before they go into service. It is also important to consider self-discharge after the battery is installed, as over a span of more than 10 years capacity losses of up to 30% are possible.
The Passivation Challenge
Although the Li-SO[Cl.sub.2] cell chemistry offers significant advantages in AMR applications, it does have one drawback for high-pulse applications. This is due to a phenomenon known as "passivation." This is because the metallic lithium in the cell rusts, just like iron, when in contact with the liquid thionyl chloride, producing a thin "passivation" layer that protects the lithium from further reaction.
Under normal conditions, this layer does not degrade cell performance. Indeed, it performs a very useful function as it prevents major loss of the cell capacity, known as "self-discharge," while the cell is in storage--resulting in a long shelf life. It also contributes to a long service life.
However, should the passivation layer grow too thick, then the cell's discharge performance may be affected. The growth of the layer is influenced greatly by storage conditions and long periods of inactivity at elevated operating temperatures will cause it to grow in thickness. A passivated cell may exhibit voltage delay, which is the time lag that occurs between the application of a load on the cell and the voltage response. As the passivation layer thickens, the voltage delay becomes more severe, and this could affect the operation of the AMR.
An important element of the expertise of the battery manufacturer, and a strong differentiation factor regarding the in-service performance of the product, lies in the correct selection of the special additives that may be added to the cell's liquid electrolyte to regulate passivation. These additives result in the formation of a useful protection layer during storage, which peels off swiftly when current pulses are applied.
There are also two other possible solutions to passivation. One is to work with the AMR designer to implement special protocols that reduce the periods of cell inactivity by calling for more regular high current pulses, hence reducing the growth of the passivation layer. The second approach is to fit a capacitor in order to power the high current pulses. It is recharged by the battery, in advance of the next pulse, and therefore eliminates the passivation drawbacks.
A Life-Time Decision
Making the right choice of battery calls for the AMR designer and installer to work in partnership with the battery supplier to ensure that all of the key factors--base current, pulse currents, cut-off voltage, temperature and environmental conditions--are considered. This is especially important as most AMR installations call for a battery service life of at least five years, while many call for more than 10 years and some now look for a battery that will last as long as the meter--15 to 20 years.
This extended service life means longer exposure to harsh environmental conditions, so it is vital to ensure that the battery will survive. Accelerated moisture and salt-spray testing can sometimes reveal some nasty surprises with rusting battery cans and connection tabs!
By Antoine Brenier, Saft Groupa, Paris, France
Antoine Brenier is business development manager for Saft's Lithium Battery Division.
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|Title Annotation:||automated meter reading|
|Publication:||Pipeline & Gas Journal|
|Date:||Jul 1, 2008|
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